It's essential to \"follow\" the pawn, so that the rook cannot move.
48. e7a7 White could still go back with the rook and execute the plan analyzed before. 48... f6f5  49. d5c4  49... g8h8 This is a theoretical draw. White has no winning plan because the king can't hide from the checks and the rook cannot be improved. 50. a5a6 